A demand of miniaturization is increasingly grown in the solid-state image sensor lens used in mobile devices such as the digital camera and the cellular phone with the imaging function and in the scanning optical system used in printing devices such as the laser printer. Therefore, a demand of the miniaturization is also increased in the imaging optical system used in these devices. Examples of a method of miniaturizing the imaging optical system includes a technique of decreasing the number of lenses, a technique of shortening a distance between the lens and an image surface, and a technique of increasing an angle of view. However, in such techniques, there is generated a problem that a field curvature or astigmatism is increased.
Conventionally, in the method of decreasing the field curvature or astigmatism, for example, a shape of lens in the imaging optical system is optimized, the number of lenses is increased, and glass whose variations in refractive index and variance are large is used as a lens material.
However, in the conventional method, because a structure of the imaging optical system becomes complicated, the demand of the miniaturization cannot be satisfied, and cost is increased from the viewpoints of material and forming.
In an image reading apparatus such as an image scanner, a copying machine, and a facsimile, there is proposed an image reading apparatus in which the astigmatism is well corrected by providing an optical member whose vertical refractive power is rotationally asymmetric in relation to an optical axis in an optical path between an imaging system and image reading means (for example, see Japanese Patent Application Laid-Open (JP-A) No. 5-14602). In an image-reading imaging lens which images information on original image onto the image reading apparatus, there are proposed an imaging lens in which at least one surface in plural surfaces constituting the imaging lens has the refractive power is rotationally asymmetric in relation to the optical axis and an image reading apparatus with the imaging lens (for example, see JP-A No. 2000-171705). However, in the image reading apparatus proposed in JP-A No. 5-14602, it is necessary to arrange the new optical member in the optical path, which results in the problems that the entire apparatus is enlarged and the number of adjustment items is also increased during assembly. In the imaging lens and image reading apparatus proposed in JP-A No. 2000-171705, because the lens having a rotationally-asymmetric refractive index distribution is used, when the optical axis is set to a Z-axis, it is necessary to combine an X-axis lens and a Y-axis lens, which results in the problem that man-hour is increased during the assembly.
There is also proposed an image-pickup lens in which at least one surface is formed in a Fresnel surface such that the miniaturization can be achieved while the field curvature is suppressed (for example, see JP-A No. 2002-55273). However, the image-pickup lens has insufficient function from the standpoint of the decrease in astigmatism or field curvature.
In designing the compact imaging optical system used in the above applications, it is preferable that a meridional image surface onto which a meridional ray is imaged and a sagittal image surface onto which a sagittal ray is imaged be brought close to an ideal image surface (design image surface) which is of a plane perpendicular to the optical axis as much as possible.
FIG. 8 shows positions of the meridional image surface and sagittal image surface in the case of the one to three-lens configurations. In FIG. 8, a coordinate of a horizontal axis indicates the position in an optical axis direction, and a coordinate of a longitudinal axis indicates the position in an image height direction. Because the lens having rotational-symmetry in relation to the optical axis is used, the rotationally-symmetric meridional image surface and sagittal image surface are obtained when the position in the image height direction corresponding to the position in the optical axis direction is obtained. In the imaging optical systems having the one to three-lens configurations of FIG. 8, the meridional image surface shown by a dashed line and the sagittal image surface shown by a solid line are designed so as to be brought close to the ideal image surface (design image surface), shown by a longitudinal axis, which is of a plane perpendicular to the optical axis as much as possible.
FIGS. 5 to 7 show optical paths of the imaging optical systems having one to three-lens configurations.
As shown in FIG. 5, in a first conventional example, the imaging optical system includes one lens and one glass plate. The ray passing through a diaphragm from an object is transmitted through a first lens 1 and a glass plate 4 to reach a sensor surface 5. An incidence surface and an exit surface of the first lens 1 and an incidence surface and an exit surface of the glass plate 4 are referred to as second to fifth surfaces respectively. The second and third surfaces are defined by a single aspherical equation. The third surface includes DOE.
As shown in FIG. 6, in a second conventional example, the imaging optical system includes two lenses and one glass plate. The ray passing through the diaphragm from the object is transmitted through the first lens 1, a second lens 2 and the glass plate 4 to reach the sensor surface 5. The incidence surface and exit surface of the first lens 1, an incidence surface and an output plan of the second lens 2, and the incidence surface and exit surface of the glass plate 4 are referred to as second to fifth surfaces, eighth surface, and ninth surface respectively. The second to fifth surfaces are defined by a single aspherical equation. A diffractive element (diffraction grating, DOE) is provided in the fifth surface, i.e., the exit surface of the second lens 2 to correct chromatic aberration.
As shown in FIG. 7, in a first comparative example, the imaging optical system includes three lenses and one glass plate. The ray passing through the diaphragm from the object is transmitted through the first lens 1, the second lens 2, a third lens 3, and the glass plate 4 to reach the sensor surface 5. The incidence surface and exit surface of the first lens 1, the incidence surface and output plan of the second lens 2, an incidence surface and an output plan of the third lens 3, and the incidence surface and exit surface of the glass plate 4 are referred to as second to ninth surfaces respectively. The second to seventh surfaces are defined by a single aspherical equation. The diffractive element (diffraction grating, DOE) is provided in the fifth surface, i.e., the exit surface of the second lens 2 to correct the chromatic aberration.
Returning to FIG. 8, in the imaging optical system, as the number of lenses is increased to bring the meridional image surface and the sagittal image surface close to the ideal image surface, the curve (dashed line) indicating the meridional image surface and the curve (solid line) indicating the sagittal image surface are bent in the periphery of the straight line (longitudinal axis) indicating the ideal image surface. That is, the curve (dashed line) indicating the meridional image surface and the curve (solid line) indicating the sagittal image surface have convex portions on an image side and an object side of the straight line (longitudinal axis) indicating the ideal image surface. Particularly, the curve (dashed line) indicating the meridional image surface has the remarkable convex portions on the image and object sides of the straight line (longitudinal axis) indicating the ideal image surface.
Conventionally, it is difficult that the meridional image surface having the remarkable convex portions on the image and object sides is brought close to the ideal image surface perpendicular to the optical axis while the number of lenses is kept constant.